Hydrodynamic performance study on a dual-caudal fin

Jet propulsion and undulatory propulsion have attracted researchers' attention widely due to their excellent hydrodynamic performance among the various propulsive patterns of aquatic animals. An attempt to combine the jet propulsion and undulatory propulsion is conducted in this paper by coming up with a model of dual-caudal fin. When the two flexible caudal fins move in the same direction, undulatory propulsion is achieved, while the two fins move in the opposite direction, jet propulsion is achieved. The hydrodynamic performance of the two propulsions is numerically investigated by fluid-structure interaction. The propulsive performance of three kinds of fin flexibility of caudal fin in the two propulsive types is firstly studied, their thrust forces, lift forces and fluid structures are compared. And then the kinematic parameters are discussed detailed including the phase difference and duty cycle. It is found that the jet propulsion obtains larger thrust force peaks which are beneficial for transient burst while the thrust force of undulatory propulsion is smoother. It is also found that the R-R case and R-M case present similar performance in thrust force which is better than R-F case in jet propulsion while R-M case performs the best in undulatory propulsion and the R-F case could obtain the largest efficiency in both of the two propulsive types. Moreover, the thrust force of jet propulsion is closely related with the duty cycle and fin flexibility in which shrink inward (instroke) quickly in R-M case could produce larger transient thrust force peaks without drag force peaks which may guide the control strategy of jet propulsion. These results will be useful for the development of flexible underwater robot.